The development of spinning wafer inspection systems has been an important tool as wafer sizes increase and maintaining throughput becomes increasingly challenging. For most equipment designs, throughput scales roughly linearly with inspected area. The use of the spinning inspection system which employs a spiral scan in place of earlier linear scans permits the most complete scan of the wafer without wasting inspection time. Furthermore, scanning data is output as a single data stream, which simplifies data processing.
As the technology of spinning wafer inspection systems evolves, higher sensitivity defect detection is desired. Higher sensitivity is achieved with shorter wavelength illuminating light and using smaller spot size. This results in a smaller depth of focus, i.e., the sample goes out of focus more easily. FIG. 1 shows a graph of DOF vs. spot size for three different illumination wavelengths. However, the inherent flatness of the wafer and the chuck may not be within the target depth of focus (the defect detection tool is required to be able to scan wafers with non-flatness of 100 um), therefore the surface of the wafer will not remain in focus and maximum sensitivity cannot be achieved unless a refocusing method is used.
Methods of keeping the laser beam in focus with the spinning wafer include: 1. Moving a beam focusing optical element attached to a servomechanism; and 2. Moving the target surface along the beam to bring it into focus. Both of these aforementioned methods suffer from very slow response time. Typical working frequencies are in the range of 0-100 Hz, which allows correction of disturbances on the order of 10 Hz. The limitation is usually defined by the size of the focusing element. This response time is far below what is needed for state of the art defect detection systems, where wafer spinning is performed at frequencies up to 10000 RPM. A high frequency auto focus system for laser beam scanning devices, to compensate for wafer and chuck motion, would be an important advancement. Correction for slow drift in laser beam position and shape, caused by the laser, the optics, or the mechanics, would also be important.